1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which the legibility and a liquid crystal display element are more easily made by combining the filter for displaying in color, a display unit and plates for effecting the transflection of light with one another.
2. Description of the Prior Art
Liquid crystal display devices are composed of the liquid crystal display element and polarizer, the liquid crystal display element having a liquid crystal material sandwiched between two glass plates. Those two glass plates are thinly coated with transparent electrodes. If a voltage used as a displaying signal, is applied across those electrodes, then the optical axis of the liquid crystal material has the property that it becomes parallel to an electric field generated by that voltage. Also in the absence of an electrical signal, the optical axis of the liquid crystal material is parallel to the two transparent electrodes and its axial direction is twisted just through 90 degrees between the two electrodes. Such a liquid crystal display element is normally called a TN(Twisted Nematic) type liquid crystal. By providing polarizers outside of said two glass plates, light can only transmit through (or be intercepted by), those display pattern portions desired to effect the display.
The liquid crystal display device is possible to effect various displays for the reasons that the transmission or interception of light is selectively effected and still with an external electrical signal. Since the legibility of a pattern to be displayed is due to external light, it is called a light receiving type display element. The light receiving type display element requires necessarily external light. To this end, the display element is used by disposing an illuminant adjacent thereto or by effectively utilizing surrounding light. With reference to conventional examples shown in FIG. 1 through FIG. 3, the respective operations thereof will be described hereinafter. FIG. 1 is a sectional view illustrating the passage of light with a liquid crystal display device used in the reflection mode. (1) is a liquid crystal display element, (2) and (3) are polarizers, and (4) is a reflector from which light is well reflected. Upon incident light L.sub.1 falling on the polarizer (2), that portion of light capable of passing therethrough has only a constant component of polarized light while the other component of polarized light is absorbed by the polarizer (2). Light passed through this polarizer (2) travels while it retains its direction of polarization identical to the optical axis of the liquid crystal in the process in which it passes through the liquid crystal display element (1) and on that portion of the liquid crystal not applied with an electric field. In the TN type liquid crystal, the optical axis of the liquid crystal is twisted just through 90 degrees between the two electrodes. Thus, when light incident upon the liquid crystal display element (1) leaves the latter, it changes to light having a direction of polarization twisted through 90 degrees with respect to the incident light. Assuming now that the polarizer (3) has an absorption axis orthogonal to that of the polarizer (2), the abovementioned incident light L.sub.1 passes through the polarizer (3) as it is left intact and reaches the reflector (4). Light reflected from the reflector (4) again passes through the polarizer (3), the liquid crystal display element (1) and the polarizer (2) and can be observed as reflected light L.sub.2 resulting in our eyes viewing the surface of the reflector (4) as being light. With an electric field applied to the liquid crystal display element (1), that portion of the liquid crystal having the electric field existing thereon has the optical axis parallel to the electric field. That is, the optical axis of the liquid crystal changes from its state in which it is parallel to the transparent electrodes to its direction perpendicular thereto. At that time when the incident light L.sub.1 passes through the liquid crystal display element (1), the same reaches the polarizer (3) with its direction of polarization remaining intact as it has left the polarizer (2) but not affected by the liquid crystal. Since the polarizers (2) and (3) have the absorption axes orthogonal to each other as described above, the incident light L.sub.1 is nearly absorbed by the polarizer (3) and does not reach the reflector (4). Of course, our eyes can view nothing and that portion of the liquid crystal display element (1) applied with an electrical signal is viewed as being black in contrast to the background formed of bright light from the reflector (4). Thus, a displayed pattern can be visually recognized. The foregoing has been described in conjunction with the polarizers (2) and (3) orthogonal to each other but, with the absorption axes of the polarizers (2) and (3) parallel to each other, the background is black and a displayed pattern is light which is reversed from the foregoing.
FIG. 2 is a section view of a liquid crystal display device in the transmission mode in which the reflector (4) in FIG. 1 is omitted. By controlling a transmission of the incident light L.sub.3 from a light source (5) in the liquid crystal display element (1) and viewing transmitted light L.sub.4 it is possible to visually recognize a displayed pattern according to the principles quite identical to those described for FIG. 1. In this case, a display can be visually recognized even in the absence of external light. This is because the light source (5) is included in the device.
FIG. 3 is a sectional view of a liquid crystal display device in the transflective mode having the features of both the reflection mode of FIG. 1 and the transmission mode of FIG. 2. (6) is a transflective plate which may be composed of what utilizes the diffuse refection caused from the surface scattering as in cloth, light diffusing paper, milkwhite resins, etc, and the diffuse transmission due to the fact that light propagates through a material while effecting the multiple scattering, or what includes a diffuse reflecting surface somewhat transmitting light such as a light transmissive material with a scattering surface coated with a thin metallic film, etc.
Such display devices have merit in that the legibility can be maintained well regardless of a quantity of external light. This is because the display devices are put in the reflection mode utilizing the incident light L.sub.1 for strong external light and in the transmission mode utilizing the transmitted light L.sub.3 from the light source (5) for weak external light. This transflective mode is widely employed as having both the advantages of light emitting type display devices and those of light receiving type display devices which are opposite to the former devices in position concerning whether the legibility is good or bad with respect to an intensity of external light.
However although liquid crystal display devices utilize the polarization of light, it is not comparatively discussed as an important factor that the circumference of the liquid crystal display element, and more particularly any reflector other than the polarizer changes a degree of polarization. Especially in the reflection mode, a cause results for which the quality of displays decreases unless there is considered the fact that, because the liquid crystal display element transmits light twice, the degree of polarization reduces on the reflecting plate.
In the information society, however, the importance of information display rises increasingly and it can not be said that complicated information is viewed more accurately and correctly. One of things to be aimed at in the future is to improve the legibility resulting from color displays and the other thereof is to increase a contrast ratio of a displayed pattern.
For an increase in pieces of display information, a large-scaled liquid crystal display element may be used thereby to utilize a multitude of coloring matters. For example, as shown in FIG. 4, (1) is a liquid crystal display element, (2) and (3) are polarizers respectively and portions enclosed with dotted line are display portions (7A) through (7D).
In the reflection mode and the transmission mode as described above, the polarizers (2) and (3) are affixed to the entire front and rear surfaces of the liquid crystal display element (1), one for each of the front and rear surfaces. Thus, with color displays effected, it is possible to use a color polarizer but only a single color can be expressed.
In order to solve this disadvantage, there is first proposed a liquid crystal display device shown in FIG. 5. A difference between that device and the conventional device shown in FIG. 4 is to affix color polarizers (2A), (2AA), (2B), (2C), (2CC) and (2D) in splits along the liquid crystal display portions (7A) through (7D) in place of the polarizer (2). (8) are border lines between the polarizer (2A) and the polarizer (2AA), between the polarizer (2C) and the polarizer (2CC), between the polarizer (2C) and the polarizer display portions (7) is good for the liquid crystal display portion (7B) of a simple pattern.
By affixing the general color polarizers along the liquid crystal display portions (7) in this way for the polarizer (2), the disadvantages resulting from the embodiment of FIG. 4 can be solved. However, the operation of affixing the polarizer (2) having several kinds (several colors) along the liquid crystal display portion (7) is only good for the liquid crystal display portion (7B) of a simple pattern. For example, with the liquid crystal display portions (7C) and (7D) having curve portions or with a complicated pattern displayed in color by the liquid crystal display, it is difficult to perform the operation of affixing several kinds of polarizers (2) with a high accuracy resulting in a reduction in productivity (i.e. manufacturing yield).
Presently color polarizers are red, green, blue, orange and yellow and have many problems. Thus, they are not widely used with practical display devices. One of the problems is that their color tones are restricted so that color tones suitable for various displays can not be obtained. Another problem is that color polarizers have contrast ratios which are particularly bad as compared with neutral polarizers, resulting in the deterioration of the legibility. Also it can not be said that the reliability is not sufficient in relation to the stability of coloring material. Due to those disadvantages, they have not been utilized with liquid crystal display devices in comparison with their being widely utilized with other display devices in which the color display has better legibility. Particularly, the orange and yellow colors do not reach the practical level.